Yes you can see the Hoover Dam for free.
You can walk or drive across the Hoover Dam as well as the Colorado River Bridge for free and see the Hoover Dam that way for free.
There's no charge for driving across or walking across the Hoover Dam or Colorado Bridge.
You only need to pay a fee to see the Hoover Dam if you want a tour of the Hoover Dam and to see the visitor center of the Hoover Dam.
There's also a charge for parking on the Hoover Dam.
The cost is $40.00 to see the Hoover Dam through a tour but that is for one person.
The cost is $10.00 for parking and then $30.00 per person for a 1 hour guided tour which includes seeing the visitor center.